456 NINETEENTH CENTURY. FT. in. 



which transparent sections of rock can be examined under 

 the microscope, and this, together with the great advance 

 in our knowledge of crystals and their growth, has brought 

 back the study of the mineralogical history of geological 

 formations. Petrologists, or students of rock-masses, are 

 now often able to determine not only the species of 

 minerals contained in any given rock, but also the changes 

 which these minerals have undergone since they were first 

 formed. This naturally is a great help in studying the 

 history of a formation, especially in cases of igneous 

 rocks ; and while it enables geologists on the one hand to 

 distinguish fresh volcanic products from those which have 

 since undergone great pressure, it assists them also in 

 explaining the nature of the more strictly metamorphic 

 rocks, such as schists and gneiss, about which there has 

 been so much dispute. 'Petrology' has, in fact, become 

 almost a science of its own, which, together with 

 ' Palaeontology,' or the study of fossil animals, may in time 

 enable us to read with comparative certainty the history of 

 the chief formations of the globe. 



Chief Works consulted. "Ly ell's 'Principles of Geology,' 'Elements 

 of Geology,' 'Antiquity of Man;' Lubbock's 'Prehistoric Times;' 

 'Quarterly Geological Journal,' vol. xxx. : Obituary of Agassiz; Judd, 

 Ann. Address to Geol, Soc. 1887. 



